<sys/errno.h>: Use __BSD_VISIBLE instead of !_POSIX_SOURCE.

<sys/errno.h>: Use __BSD_VISIBLE instead of !_POSIX_SOURCE.

_POSIX_SOURCE is no longer set since it was superseded by
_POSIX_C_SOURCE. In other words, #ifndef _POSIX_SOURCE is
always true, unless IEEE Std 1003.1-1988 is explicitly
requested.

While here, clean up the visibility of all errno defines.
Those not mentioned in any standard are __BSD_VISIBLE now.
We can further restrict the standard ones at some later
point.